1,720,983 research outputs found
Reagent control in the aldol addition reaction of chiral boron enolates with chiral aldehydes. Total synthesis of (3S,4S)-statine
No full textBoron enolates bearing menthone-derived chiral ligands are capable of fair to excellent diastereocontrol in their reactions with chiral aldehydes. Thioester-derived (better than ketone derived) enolates are able to control aldol stereochemistry irrespective of the aldehyde preferences. With thioacetate-derived chiral enolates and enantiopure N,N-dibenzyl alpha-amino aldehydes, either the 3,4-anti or the 3,4-syn aldol adduct can be obtained with very high diastereoselectivity just by changing the chiral boron ligand configuration. The above procedure was used for a stereoselective total synthesis of (3S, 4S)-statine. (C) 1997 Elsevier Science Ltd
A HIGHLY ENANTIOSELECTIVE AND DIASTEREOSELECTIVE ALDOL REACTION FOR ALPHA-HETEROSUBSTITUTED THIOACETATES
No full textBoron enolates derived from alpha-heterosubstituted thioacetates and bearing menthone-derived chiral ligands react with aldehydes to give anti aldols with excellent diastero- and enantiocontrol
Highly enantio- and diastereoselective boron aldol reactions of alpha-heterosubstituted thioacetates with aldehydes and silyl imines
No full textBoron enolates derived from alpha-heterosubstituted thioacetates and bearing menthone-derived chiral ligands react with aldehydes to give anti aldols with excellent diastero- and enantiocontrol. Boron enolates derived from tert-butyl alpha-halothioacetate and bearing menthone-derived chiral ligands react with imines with excellent diastero- and enantiocontrol to give syn alpha-halo-beta-aminothioesters, which can be converted to the corresponding aziridines by simple ring closure during LAH reduction. A key precursor of antibiotics (+)-thiamphenicol and (-)-florfenicol was synthesized. (C) 1997 Elsevier Science Ltd
REAGENT CONTROL IN THE ALDOL ADDITION-REACTION OF CHIRAL BORON ENOLATES WITH CHIRAL ALDEHYDES
No full textBoron enolates bearing menthone-derived chiral ligands are capable of fair to excellent diastereocontrol in their reactions with chiral aldehydes. Thioester-derived (better than ketone derived) enolates are able to control aldol stereochemistry irrespective of the aldehyde preferences
Computer‐Assisted Design of Chiral Boron Enolates: A Novel, Highly Enantioselective Aldol Reaction for Thioacetates and Thiopropionates
No full textUnsubstituted and anti aldol products with excellent diastereo- and enantioselectivity are formed when enolates, generated from the corresponding thioacetates and thiopropionates by using bromoborane 1 and triethylamine, are treated with aldehydes. Bromoborane 1 was designed based on transition state computer modeling and then synthesized
ORIGINS OF STEREOSELECTIVITY IN THE ADDITION OF ALLYL AND CROTYLBORONATES TO ALDEHYDES - THE DEVELOPMENT AND APPLICATION OF A FORCE-FIELD MODEL OF THE TRANSITION-STATE
No full textA molecular mechanics model of the transition state for the addition of allyl and crotyl boronates to aldehydes was developed, based on ab initio calculations and on a process of vial and error optimization. The optimized force field reproduces the experimental syn-anti stereoselectivity for the intermolecular addition of E and Z crotylboronates to aldehydes. The force field is used to analyse the stereoselectivity of various synthetically interesting reactions. In particular, the force field is able to reproduce with excellent quantitative agreement the experimental results of intramolecular boronate reactions leading to methyl or benzyloxy substituted cyclohexanol or cyclopentanol derivatives (R.W. Hoffmann, et al.). In addition the force field is able to reproduce the experimental results of variously substituted allylboronates (gamma,gamma-alkyl substituted, alpha,gamma-alkyl substituted, alpha,gamma,gamma-alkyl substituted, gamma-alkoxy substituted) and the Felkin-antiFelkin ratios of allyl, E-crotyl and Z-crotyl addition to chiral aldehydes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Semisynthesis of taxol: A highly enantio- and diastereoselective synthesis of the side chain and a new method for ester formation at C13 using thioesters
No full textTaxol semisynthesis: A highly enantio- and diastereoselective synthesis of the side chain and a new method for ester formation at C-13 using thioesters
No full textA very simple, new, and straightforward approach to the Paclitaxel (Taxol) and Docetaxel (Taxotere) side chains has been developed using the imine addition reaction of thioester-derived boron enolates bearing chiral ligands. The addition reaction was studied extensively, using a combination of different thioesters (ROCH2COSPh, ROCH(2)60St-Bu), oxygen protecting groups (R = Bn, TBDMS, COPh, EE, TMS), chiral boron ligands [derived from both (-) and (+)-menthone], imines (PhCH=NSiMe3, PhCH=NCOPh), and in the presence or in the absence of additional Lewis acids (BF3-OEt2, Et2AlCl, TiCl4). The side chain was assembled in a few steps with the correct relative (syn) and absolute stereochemistry (2R,3S). The stereochemical outcome of the boron-mediated reaction was rationalized using chair vs boat transition state structures. A new direct route for attachment of the side chains to the baccatin nucleus using thioester chemistry has also been developed. By treatment of a mixture of a thioester (8, 12, or 17) and protected baccatin III (2b, 2c) with LHMDS, the 13-O acylated compounds were obtained in high yield (up to 90%). Hydrolysis of 18b gave Paclitaxel (1a) in 80% yield
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